Two basic factors that limit the performance of optical fiber communication systems are chromatic dispersion and nonlinear effect of an optical fiber link. For instance, for a 10 Gbps optical fiber communication system, when the launched power of optical signal is 0.0 dBm, the transmission distance of a standard mono-mode optical fiber is about 60 km if the chromatic dispersion is not compensated, while the transmission distance for a 40 Gbps optical fiber communication system is only 3.5 km. In the case that the launched power is higher than 0.0 dBm, the signal distortion that the nonlinear effect produces allows for a shorter transmission distance of the optical fiber communication system. Therefore, for an optical fiber communication system or an optical fiber link, it is necessary to monitor in real-time and equalize the chromatic dispersion and the nonlinear effect of the optical fiber.
Methods of measuring the chromatic dispersion of an optical fiber mainly include a frequency spectrum analysis method, a vestigial sideband filter method, a nonlinear spectral analysis method and the like. These methods may intrude the operation of an optical fiber link, and are not transparent to the modulation formats and data rate, thus cannot satisfy the requirement of monitoring the practical optical fiber links in real time. Moreover, a measurement of the nonlinear effect of an optical fiber is still a world-famous puzzle, and no method of quantitative measurement has yet been currently proposed. As the data rate of an optical fiber communication system is increasingly higher, and the channel spacing is increasingly narrower, it becomes more and more important to estimate and equalize the chromatic dispersion and the nonlinear effect of an optical fiber. Therefore, how to perform an intrusion-free, real-time, and accurate monitoring of the chromatic dispersion and the nonlinear effect in an ultrahigh speed optical fiber optic communication link is a difficult problem urgently required to be solved in the field of optical communications.
In an optical fiber communication system or network, the traditional method for measuring the nonlinear effect in the optical fiber link mainly includes an all-optical pre-processing method, a frequency spectrum analysis method, a binarization method, a nonlinear spectral analysis method and the like (see “research on method for digital data processing in optical fiber detection”, infrared and laser engineering. Vol. 30, No. 3, LIU Yange et al, June, 2001). The above traditional Fourier transformation based methods for identifying and measuring the nonlinear effect of optical signal are fast. But the resolution and accuracy are low, the Fourier transformation based methods are not suitable for an optic communication system of high speed and consisting of different types of optical fibers.
Following retrieval, none of the domestic and foreign literatures and patents report the method based on a FRFT for monitoring the nonlinear effect in an optical fiber link. And how to accurately monitor the nonlinear effect of an optical fiber link in real-time and intrusion free is a difficult problem to be urgently solved in the field of ultrahigh speed optical fiber communications.